1. Field of the Invention
The invention is related to NMR logging techniques in a downhole environment in petrophysical testing. In particular, the invention quantifies the effects of borehole size, formation resistivity and invasion on measurements that can be made with a NMR tool.
2. Description of the Related Art
Nuclear Magnetic Resonance (NMR) has uses in many areas, including the fields of medicine, chemistry, non-destructive testing, and in well logging in the oil exploration industry. In the well logging industry, NMR wireline logging or measurement-while-drilling (MWD) instruments are useful for collecting information on earth formation properties and for characterizing reservoir fluids. NMR is used in determining properties such as porosity of the formation, permeability, the movable fluid volume (BVM), the clay bound volume (CBW) and bulk volume irreducible (BVI), as well as other formation and reservoir fluid properties.
In a typical NMR device used in logging, a permanent magnet produces a static magnetic field and establishes a direction of orientation for nuclear magnetic moments in the vicinity of the borehole. An RF field is applied in the plane perpendicular to the static magnetic field. Typically in the art, the static field B0 is a function of distance from the tool. Thus, at a given applied frequency, the NMR resonance condition must be satisfied, wherein
                    f        =                              γ            ⁢                                                  ⁢                          B              0                                            2            ⁢            π                                              (        1        )            where f is the frequency of the RF field, and γ is the gyromagnetic ratio. Nuclei that are influenced by the applied RF field typically lie within a certain volume, named the sensitive volume. For a selected operating frequency, the location and size of the sensitive volume are determined by the magnetic field intensity, the field gradient and the effective bandwidth of the pulse. In multi-frequency logging, a discrete number of closely spaced and substantially non-overlapping sensitive volumes can be obtained. The union of these sensitive volumes is defined as the region of examination of a given tool with a given acquisition method.
In centralized tools, the region of examination is a cylindrical shell which is coaxial with the permanent magnet, although other spatial arrangements can be used. Since the region of examination typically lies close to the surface of the borehole cavity, a perfectly coaxial alignment of the tool and borehole wall, in which the borehole wall is circular and smooth, would yield optimal values of echo signals. Occasionally though, geometric anomalies concerning the logging tool and the surface of the borehole will result in portions of the region of examination lying inside the borehole cavity rather than inside the rock formation. As one example of possible anomalies, the tool can be off-axis with the borehole and additionally can be lying against one side of the borehole, revealing a portion of the region of examination to the borehole cavity. In another example, the borehole might have an elliptical cross-section rather than a circular one. In yet a third possibility, there can be a significant amount of washout, where certain segments of the wall have separated and fallen away, leaving a cavity to one side of the borehole.
Measurements made with NMR logging instruments, being electromagnetic measurements, are responsive to a greater or lesser degree, on other formation and borehole properties besides those related to nuclear spins. This is recognized in U.S. Pat. No. 5,828,214 to Taicher et al, having the same assignee as the present invention and the contents of which are incorporated herein by reference. Taicher teaches an NMR apparatus in which the frequency of the RF field is varied. It is shown that the phase difference between the RF field and the resonance signal depends upon the conductivity of the path over which the RF field and the signals travel. By measuring phase differences at two different frequencies, it is possible to determine the conductivity as a function of radius.
NMR signals are also affected by the borehole size. Drilling mud is typically used to facilitate drilling, and therefore yields a constant presence within the borehole. Typically, drilling mud is either oil-based (including synthetic oil-based), water-based, or glycol-based and hence has a large number of hydrogen nuclei. Due to the large number of hydrogen nuclei, the mud is a strong source of contamination in NMR spin echo signals, and the contamination signals can be greater than the desired signals obtained from the rock formation. U.S. patent application Ser. No. 10/855,230 of Chen et al. having the same assignee as the present application and the contents of which are incorporated herein by reference uses echo signals acquired from a plurality of different regions of investigation at different depths of investigation. The echo signals are analyzed to obtain an indication of possible presence of a borehole fluid in at least one of the regions of examination.
The methods discussed by Taicher and by Chen thus provide information about the formation resistivity and borehole size, something that is different from the main objective of determining formation porosities and relaxation time distributions. They both rely on the use of relatively weak NMR signals to estimate the parameters of interest. It would be desirable to have a method and apparatus that uses stronger signals to determine the same parameters. The present invention satisfies this need.